1. Field of the Invention
The present invention relates to a radiation image read-out apparatus, and more specifically, to a radiation image read-out apparatus for reading an image recorded on a stimulable phosphor sheet using a line sensor.
2. Description of the Related Art
Heretofore, there have been widely used radiation image recording and reproducing systems utilizing stimulable phosphors. Specifically, a radiation image of an object (e.g., a human body) is recorded on a stimulable phosphor sheet, which includes a substrate and a layer of the stimulable phosphor overlaid on the substrate. A beam of stimulating light (e.g., a laser beam or a beam of visible light) is variably deflected to scan individual pixels of the radiation image recorded on the stimulable phosphor sheet. The beam of the stimulating light causes each pixel to emit stimulated emission light in proportion to the amount of radiation energy stored thereon. The light emitted successively from the individual pixels of the radiation image recorded on the stimulable phosphor sheet is photoelectrically detected and converted into an electric image signal by photoelectric read-out means. After the entire surface of the stimulable phosphor sheet is scanned, the stimulable phosphor sheet is exposed to erasing light so that the radiation energy remaining thereon is completely released.
The image signal, which has been obtained by the radiation image recording and reproducing system, is then subjected to image processing. The image processing may include gradation processing, processing in the frequency domain, etc., for reproducing the radiation image in a visible form having image quality high enough to serve as an effective tool in conducting efficient and accurate diagnosis of a diseased portion. The visible image for diagnosis reproduced from the image signal may be printed on a film or may be displayed on a high resolution cathode ray tube (CRT) display device. After the erasing light releases the residual radiation energy on the stimulable phosphor sheet, the stimulable phosphor sheet may be reused for recording of another radiation image.
Novel radiation image read-out apparatuses for use in the radiation image recording and reproducing systems as described above have been proposed in, e.g., Japanese Unexamined Patent Publications Nos. 60(1985)-11568, 60(1985)-236354 and 1(1989)-101540. The radiation image read-out apparatuses in the above listed publications are directed to shortening the time required for detecting the stimulated emission light, to downsizing the apparatus, and to realizing a lower operation cost. To achieve those objects, each of the proposed radiation image read-out apparatuses includes a linear light source as the stimulating light source for irradiating the stimulable phosphor sheet with a linear beam of the stimulating light, and a line sensor as the photoelectronoc read-out means having a plurality of photoelectronic conversion elements aligned parallel to a linear beam spot of the stimulating light on the stimulable phosphor sheet. Each of the proposed radiation image read-out apparatuses also includes scanning means for moving the linear light source and the line sensor relative to the stimulable phosphor sheet in directions substantially perpendicular to the linear beam spot on the stimulable phosphor sheet.
The proposed radiation image read-out apparatus utilizing the line sensor may further comprise an array of lenses providing certain refraction index distribution, e.g., an array of SELFOC lenses (registered trademark) or rod lenses, to sufficiently focus the stimulated emission light emitted from the stimulable phosphor sheet onto the line sensor. Such an array of lenses providing certain refraction index distribution realizes one-to-one correspondence between the size of the recorded image and the size of the obtained image. The individual lenses in the array are arranged in accordance with the arrangement of the photoelectronic conversion elements on the line sensor. For example, if the photoelectronic conversion elements are arranged on the line sensor as shown in FIG. 2, the individual lenses in the array will be arranged as shown in FIG. 3.
However, as is clear from FIG. 3, the array of the lenses providing certain refraction index distribution naturally includes non-aperture regions, i.e., those regions among the lenses. The non-aperture regions are lower in transmittance of the stimulated emission light than aperture regions (or lens regions). Some portion of the stimulated emission light emitted from the stimulable sheet passes the non-aperture regions while the other portion of the stimulated emission light passes the aperture regions before reaching the line sensor. Therefore, a spurious stripe pattern having a pitch corresponding to the pitch of the non-aperture regions may appear on an image reproduced from the image signal obtained by the radiation image read-out apparatus. Each stripe will be perpendicular to the length direction of the line sensor.
An object of the present invention is to provide a radiation image read-out apparatus capable of removing from a read-out image a spurious stripe pattern due to non-aperture regions of the lens array.
According to the first aspect of the present invention, there is provided a radiation image read-out apparatus comprising: a linear light source for irradiating a stimulable phosphor sheet carrying a radiation image recorded thereon with a linear beam of stimulating light; a line sensor comprising a plurality of photoelectric conversion elements arranged parallel to a linear area on the stimulable phosphor sheet irradiated with the linear beam of the stimulating light, each of said photoelectric conversion elements being capable of photoelectrically converting stimulated emission light received thereon, said stimulable emission light being emitted from the linear area on the stimulable phosphor sheet irradiated with the linear beam or from a corresponding linear area on the opposite side of the stimulable phosphor sheet; focusing means located between the stimulable phosphor sheet and the line sensor for focusing the stimulated emission light onto each of the photoelectric conversion elements, said focusing means including a lens array; scanning means for moving the linear light source and the line sensor relative to the stimulable phosphor sheet in a direction not parallel to the linear area on the stimulable phosphor sheet irradiated with the linear beam of the stimulating light; read-out means for deriving an image signal representing the radiation image recorded on the stimulable phosphor means from the electric signal outputted by the line sensor while the linear light source and the line sensor is moved relative to the stimulable phosphor sheet; and spurious pattern removing means for obtaining a processed image signal by removing from the image signal a spurious pattern signal due to non-aperture regions on the lens array.
The linear light source may be a light source having a linear shape by itself, such as a fluorescent lamp, a cold cathode fluorescent lamp or an LED array. Otherwise, the linear light source may be a light source which does not have a linear shape by itself but is capable of emitting a linear beam, such as a broad area laser. Although the linear beam of the stimulating light emitted by the linear light source may be either of a continuous beam or a pulse-like beam, use of the pulse-like beam is preferred in order to reduce resultant noise.
It is preferable to make the length of the beam spot of the linear beam of the stimulating light equal to or longer than the length of one side of the stimulable phosphor sheet. The linear area on the stimulable phosphor sheet irradiated with the linear beam of the stimulating light may be aligned parallel to the side of the stimulable phosphor sheet, or may be defined at a certain angle with respect to the side of the stimulable phosphor sheet.
It is preferable to provide between the linear light source and the stimulable phosphor sheet an optical system for focusing the linear beam of the stimulating light onto the surface of the stimulable phosphor sheet. The optical system may include a cylindrical lens, a slit, a lens array of lenses providing certain refraction index distribution, a fluorescence inducing sheet, a bundle of optical fibers, and any appropriate combination thereof. In the case where the wavelength of the desired second order stimulating light for the selected stimulable phosphor sheet is approximately 600 nm, a desired fluorescence inducing sheet is a sheet of a glass material or a polymer containing Eu3+ as a phosphor activator.
It is preferable that the linear area on the stimulable phosphor sheet irradiated with the linear beam of the stimulating light has a width of 10-4000 xcexcm.
Used as the line sensor may be, for example, an amorphous silicon sensor, a CCD sensor, a CCD with a back illuminator, or a MOS image sensor.
The lens array in the focusing means is preferably such a lens array providing certain refraction index distribution.
In addition, the lens array of the focusing means may be, for example, an array of SELFOC lenses or rod lenses. Such an array of lenses sufficiently focuses the stimulated emission light emitted from the stimulable phosphor sheet onto the line sensor by providing certain refraction index distribution which realizes one-to-one correspondence between the size of the recorded image and the size of the obtained image. Each of the lenses is made of a glass material or a polymer.
The focusing means including the lens array may further comprise additional elements for improving focusing ability thereof. Such additional elements may include a cylindrical lens, a slit, a bundle of optical fibers, and any appropriate combination thereof.
It is preferable that the radiation image read-out apparatus further comprises a stimulating light cutting filter (e.g., a sharp-cut filter or a band pass filter), which transmits the stimulated emission light but blocks the stimulating light, for preventing the stimulating light from reaching the line sensor.
A light receiving surface of each of the photoelectric conversion elements on the line sensor is smaller in each dimension than the width of a linear beam spot thereon of the stimulated emission light. A plurality of photoelectric conversion elements are arranged over a length equal to or longer than the linear beam spot of the stimulated emission light.
The line sensor may be of a multi-line configuration, i.e., may include plural lines of the photoelectric conversion elements. Four to twelve lines are especially preferred. In this case, the photoelectric conversion elements constituting the line sensor may be arranged in either of a matrix-like arrangement, an arrangement staggered perpendicular to the length direction of the line sensor, an arrangement staggered parallel to the length direction of the line sensor, or an arrangement staggered in both of the two directions. In either case, it is preferred to arrange individual lenses in the lens array in an arrangement corresponding to the arrangement of the photoelectric conversion elements on the line sensor.
In the case where the line sensor includes so many photoelectric conversion elements that the effect of read-out rate is not any more negligible, it is preferable to allot a storage element to each of the photoelectric conversion elements so that the charges generated by the photoelectric element are stored temporarily in the allotted storage element thereof.
The line sensor preferably includes more than one thousand photoelectric conversion elements per line. The length of the line sensor is preferably equal to or longer than the length of one side of the stimulable sheet.
The direction in which the scanning means moves the linear light source and the line sensor relative to the stimulable phosphor sheet is preferably the width direction of the line sensor (i.e., the direction perpendicular to the length direction of the line sensor). However, in the case where each of the lengths of the linear light source and the line sensor is larger than the length of one side of the stimulable sheet, the scanning means may instead move the linear light source and the light source in a direction fixed at a certain angle with respect to the length direction as far as the linear light source remains capable of irradiating the entire surface of the stimulable sheet. Otherwise, the linear light source and the line sensor may be moved in a zigzag motion.
The linear light source and the line sensor may be arranged either on the same side or on the opposite sides of the stimulable phosphor sheet. In the case where the linear light source and the line sensor are arranged on the opposite sides, a substrate of the stimulable phosphor sheet must be made of a material which transmits the stimulated emission light, so that the stimulated emission light induced by the linear light source is appropriately transmitted toward the line sensor.
It is preferred that the spurious pattern removing means in the radiation image read-out apparatus comprises: spurious pattern signal calculating means for calculating a spurious pattern signal representing a spurious pattern due to non-aperture regions of the lens array based on arrangement of the non-aperture regions on the lens array; a memory for storing the calculated spurious pattern signal; and image modification means for modifying the image signal using the spurious pattern signal.
The spurious pattern signal is a one-dimensional signal representing a spurious pattern in the length direction of the lens array.
It is still more preferred that the spurious pattern removing means comprises: a memory for storing a standard image signal representing an image of uniform radiation recorded on the stimulable phosphor sheet; and an image modification means for modifying the image signal based on the standard image signal.
The standard image signal is a two-dimensional signal representing a spurious pattern over the entire surface of the stimulable phosphor sheet.
Using the radiation image read-out apparatus of the present invention, the quality of the read-out image is improved as the spurious stripe pattern due to the non-aperture regions of the lens array is removed from the read-out image.
In the case where the line sensor includes plural lines of the photoelectric conversion elements, the detection efficiency of the line sensor is also improved as the entire light receiving area for the stimulated emission light is increased.
In the case where the one-dimensional spurious pattern signal is used for modifying the image signal, the entire apparatus may be realized with a relatively simple configuration as required capacity of the memory for storing the one-dimensional spurious pattern signal is relatively small.
In the case where the two-dimensional standard image signal is used for modifying the image signal, the required capacity of the memory will be relatively large. However, the two-dimensional standard image signal includes effects of unevenness of the base radiation (pixel-by-pixel unevenness), sensitivity of the stimulable sheet (pixel-by-pixel unevenness), the stimulating light (line-by-line unevenness), efficiency of the stimulated emission light reaching the line sensor (line-by-line unevenness), and sensitivity of each photoelectric conversion elements on the line sensor (line-by-line unevenness). Therefore, use of the two-dimensional standard image signal has an advantage of canceling all those effects on the read-out image.